Ethernet is increasingly popular as a transport network for high speed Wide Area Network (WAN) communications. Various techniques have been developed to enable the deployment of Ethernet transport networks with a mesh topology. Among other things, these enable the provisioning of topologically diverse paths for traffic protection, while preventing the formation of loops within the network.
Referring to FIG. 1a, Split Multi Link Trunking (SMLT), which is described in Applicant's U.S. Pat. No. 7,269,132 provides a technique by which a client system (CS) 2 can be connected to an Ethernet network domain 4 though a pair of peer Ethernet Switches (ESs) 6a-b. In very general terms, a Split Link Aggregation Group (SLAG) 8 is defined, which comprises a plurality of parallel links 10 (in FIG. 1a, the SLAG comprises two parallel links). At one end of the SLAG 8, the links 10 are connected to the CS 2 via an aggregator function 12, which serves to distributed packet traffic across the links 10 of the SLAG 8. At the opposite end of the SLAG 8, each link 10 is connected to a respective ES 6, via a corresponding instance of a distributed aggregator function 14. This arrangement provides a “dual homed” connection to the Ethernet domain 4, and facilitates load balancing and inherent resiliency. In order to prevent network instability and loops, complex algorithms must be implemented to coordinate the operations of the peer ESs 6a-b. Typically, this involves controlling the distributed aggregator function 14 and the involved peer ESs 6 to emulate a single aggregator/ES system which mirrors the CS/aggregator system 2,12 at the other end of the SLAG 8.
Referring to FIG. 1b, dual homing is also known for connecting neighboring network domains. In the example illustrated in FIG. 1b, an Ethernet domain 4 may be connected to a Provider Backbone Transport (PBT) network domain 16 via a pair of peer gateways 18a-b. Within each domain, respective processes can be used to compute paths and install traffic forwarding information to enable end-to-end connectivity. For example, spanning tree protocol may be used to compute paths in Ethernet and PBT network domains, while MPLS may be used in an IP domain. As in the case of Split Multi Link Trunking, it is normally necessary to implement complex algorithms in each of the peer gateways 18 to prevent the formation of loops.
For example, consider a scenario in which an Ethernet domain 4 is connected to a PBT network domain 16 via a pair of peer gateways 18, as shown in FIG. 1b. In each network domain, spanning tree protocol is used to compute paths and prevent the formation of loops. However, by definition, paths are computed in each domain independently. As a result, there is, in principle, nothing to prevent a path 20 between the two peer gateways 18 being set up through the Ethernet domain 4, and a second path 22 between the two gateways 18 being set up through the PBT domain 16. Within each domain, each path 20,22 is guaranteed to be loop free, by operation of STP. However, taken together the two paths form a loop which, under some circumstances, could destabilize the network. Typically, this problem is addressed by implementing special algorithms in the peer gateways 18 and/or one of the network domains, to coordinate operations of the two peer gateways 18 so as to prevent packets from looping. For example, in some cases, forwarding information in each gateway 18 may be arranged to force traffic through only one of the two gateways 18.
A simpler method that enables resilient connection of an Ethernet domain to another network domain remains highly desirable.